Power FETs are widely used in the automobile industry and are utilized to drive the solenoid valves in anti-lock brake systems (ABS), for example. In certain ABS braking systems, it is desirable to pulse width modulate (PWM) the signal to the solenoid in order to control the antilock function. This, in turn, requires that the turn on and turn off times of the FET switching transistor be less than 250 ns whereas turn on and turn off times of approximately 50 us were required in prior art ABS braking control systems.
In order to protect the FET from being damaged by an overcurrent condition, a current limit circuit is provided to control the maximum current allowed through the FET. In an ABS braking system, for example, it is possible for the solenoid coil to be shorted or have a fault which allows a high current to be drawn from the vehicle's battery when the FET switch is turned on. If it were not for the overcurrent protection circuit, the FET switch would be destroyed and, since the FET switch is integrated into the control integrated circuit, the entire ABS control circuit could be damaged or destroyed.
One might wonder why protection for the FET switch is important if the solenoid valve has failed, since the loss of appropriate braking power could be a catastrophic loss in the event it was necessary to make emergency stop with the vehicle or make a stop on slippery pavement, in which ABS braking systems excel. In order to avoid an incident in which the ABS braking system fails when it is needed, it is common for such systems to test the operation of the system at start up and/or while the vehicle is being driven. Thus, for example, the control system might apply a very short spike of 500 us or less, for example, to a solenoid valve in order to check the operation thereof. This would be undetectable to the driver of the vehicle. If the system detects that the solenoid valve is shorted, it would then turn on an indicator lamp on the dashboard of the vehicle to alert the driver that the ABS system was malfunctioning. The driver would then know not to rely on the ABS system and to take his vehicle for repair as soon as possible. In order to avoid damage to the integrated circuit containing the FET switching transistor, it is necessary to have a current limit function, which will protect the device during these tests as well as during the actual operation of the braking system.